Newton's Second Law
Newton's Second Law states that the acceleration of an object when produced by a net force is directly proportional to the force applied and inversely proportional to the mass of the object. F = ma We have investigated this through the following examples: * Toy Story 3 * The Incredibles Toy Story 3 During Toy Story 3, Woody is trying to escape from Sunnyside Day Care when his pull- string gets caught on a tree branch. Initially, woody accelerates due to gravity. Assuming Woody’s mass to be 0.5kg and using the acceleration due to gravity to be 9.81 ms-2, we can estimate the force to be around 4.91 N using Newton’s Second Law. Once Woody has fallen the length of the string, he initially bounces back up due to elastic potential energy. After a few bounces, this elastic energy is ‘used up’ due to the air resistance, causing Woody to dangle. He remains stationary, as the tension in the string acts upwards and so stops him from accelerating towards the ground. As there is now no acceleration, the net force must be 0. This means the tension in the string must be 4.91 N in order to counteract the force due to gravity. This seems like a reasonable value for an average toy string to be able to withhold. To conclude, this scene from Toy Story 3 is a good example of Newton’s Second Law in action. At first, the force is only due to gravity which causes an acceleration but by the end the string is also providing tension, causing the overall force to be 0 and so there is no acceleration, as stated by F=ma. The Incredibles At the beginning of The Incredibles, Mr. Incredible is faced with a situation where he must stop an oncoming train before it reaches a gap in the track and derails. Assuming that the train is largely similar to a New York subway train, the top speed that the train can reach is 90 km/h (or 25 m/s), its maximum deceleration due to breaking is 1.4 m/s2 and a typical 4-car service has a mass of around 160,000 kg. https://en.wikipedia.org/wiki/R142A_(New_York_City_Subway_car) Mr. Incredible grabs the train and manages to slow it down enough that it is barely moving by the time it reaches the gap in the track. Assuming that when it reaches the edge it has effectively stopped for ease of calculations, the time between the moment Mr. Incredible makes contact with the train and where the train stops is 6.3 seconds. Assuming that at the point of contact, the train is travelling at its maximum speed, the driver has just applied the emergency breaks and that there is negligible friction between the train and the tracks, the amount of force that Mr. Incredible applies (assuming that it is constant) can be found. Using Newton's Second Law, the amount of force he applies has been calculated to be 400,000 Newtons, which is comparable to the thrust supplied by some jet engines!https://en.wikipedia.org/wiki/Jet_engine Obviously, this wouldn't be possible for any normal human being, and if Mr. Incredible wasn't so...incredible, he would be easily flattened. References